[exim] 'failed to lock' delivery deferral breaks system filt…

Exim 4.89

I manage the email server for a small company in the US. Less than 1000
received messages per day. A month or two ago I put in place a simple exim
system filter that removes the 'Friendly Name' from the From: header.

headers add "X-old-from: $header_from:"
headers remove "From"
headers add "From: <${address:$header_X-old-from:}>"

This works well most of the time and has had good acceptance from the user
community as part of anti-phishing efforts.

However, if delivery is deferred to one recipient because of a lock failure
(example follows) then the final line of the filter does not execute, and
more than one (perhaps all) recipients receive a message with no From:
header. The X-old-from: header does appear in the delivered message.

... defer (-9): failed to lock mailbox /var/mail/redacted (fcntl/flock)

A pointer to a fix or workaround would be lovely; my current best
alternative is to switch from mbox to maildir to reduce the frequency of
delivery deferrals.

Thanks.

Ken